Pressure solution at the molecular scale.

The topological evolution of the cleavage surface of a gypsum single crystal during its dissolution in a flowing undersaturated aqueous solution has been observed with an atomic force microscope. The matter transfer from solid to liquid proceeds through the migration of atomic steps. The step velocity has been measured and appears to depend on the force applied by the tip on the surface. Whereas the high force velocity enhancement is likely to stem from corrosive wear, the speed behavior at low force (<10 nN) differs drastically and can be interpreted as a consequence of the pressure solution of the crystal induced by the tip force. The step velocity evolution with the force obeys the known kinetic law of pressure solution. Hence these experiments enable us to evidence a first atomic mechanism at the origin of pressure solution.

Pressure solution as origin of the humid creep of a mineral material.

The significant enhancement of the creep of plasterboard by a humid environment is well known in the building industry. But despite its strong impact on the material durability, its origin remains unexplained. We present here experimental evidence that the creep of wet set plaster is driven by the dissolution kinetics of gypsum, its major component, in intercrystalline water layers. Linking this surface molecular behavior to a macroscopic mechanical property has been made possible by the establishment, using holographic interferometry, of an accurate method of convection-free dissolution measurement, and by the possibility of tuning the dissolution kinetics of the material by the use of additives. Although it is well known in geological contexts, this dissolution-creep correlation had not yet been observed outside this field. It enables one to propose pressure solution as the mechanism of the wet creep of set plaster and sheds light on the humid creep of polycrystalline mineral materials.